The present invention relates to a device (catalytic converter) for purifying hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), which are contained in the exhaust gas from the internal-combustion engine of automobiles and boilers. The present invention relates in particular to a catalyst which is capable of purifying an oxygen rich exhaust gas by removing NOx contained in the oxygen rich exhaust gas.
In recent years, there has been a large demand for low-fuel-consumption automobiles, in view of the possibility of exhaustion of the petroleum resources and the global warming problems. In particular, there is a strong demand for lean-burn automobiles in the field of gasoline-engine automobiles. Such a lean-burn automobile is driven on the lean side with a large air-fuel ratio (A/F) as much as possible during an ordinary driving, so as to burn an oxygen rich air-fuel mixture and thereby meet with the demands for lower fuel consumption. JP-A-9-184441 discloses an air-fuel ratio control device of an internal combustion engine, which may be driven on the lean side. In case of an oxygen rich (lean) air-fuel mixture, the oxygen content of an exhaust gas after combustion becomes high. If this exhaust gas is allowed to flow through a conventional three-way catalyst, the oxidation action becomes more active and the reduction action becomes inactive. In view of this, it has been desired to provide a catalyst that can sufficiently remove NOx by reducing NOx to N2 even on the lean side, that is, in an oxygen rich exhaust gas. Japanese Patent Unexamined Publication JP-A-5-168860 discloses a catalyst having a porous carrier supporting thereon platinum and lanthanum.
If sulfur is contained in the fuel and/or lubrication oil, this sulfur may be exhausted in the form of sulfur oxide into the exhaust gas. This sulfur oxide may reduce the NOx purification capability of the above-mentioned catalyst that is capable of removing NOx. Thus, it is desired to provide a catalyst that is capable of suppressing such reduction of the NOx purification capability caused by sulfur. U.S. Pat. No. 5,472,673, which corresponds to JP-A-6-58138, discloses an exhaust gas purification device having a sulfur trapping device.
It is an object of the present invention to provide a device for purifying an exhaust gas, which is improved in capability of purifying NOx contained in an oxygen rich exhaust gas.
According to the present invention, there is provided a device for purifying an exhaust gas. This device comprises a first catalyst for purifying a NOx of the exhaust gas. This first catalyst comprises first and second powders. The first powder comprises a porous carrier and at least one noble metal loaded on the porous carrier. The at least one noble metal is selected from the group consisting of platinum, palladium and rhodium. The second powder comprises a first double oxide represented by the following general formula (1):
(Ln1-xcex1Axcex1)1-xcex2BOxcex4xe2x80x83xe2x80x83(1)
where xcex1 is a number that is greater than 0 and less than 1, xcex2 is a number that is greater than 0 and less than 1, xcex4 is a number that is greater than 0, Ln is at least one first element selected from the group consisting of La, Ce, Nd and Sm, A is at least one second element selected from the group consisting of Mg, Ca, Sr, Ba, Na, K and Cs, and B is at least one third element selected from the group consisting of Fe, Co, Ni and Mn.